Strategy

As soon as you enter the boss room, you begin in a church-like area. In front of you is a bunch of crawling hollows which are easy to dispose of. Then there are the three enemies you seriously need to worry about: the two Hollow Priests, and the Magus.

The best way to go about this is to get rid of the Priests first as they can cast an area of effect heal miracle. Watch out for their Sunlight Spears that can deal heavy damage.

Use the pews in the room to your advantage. If you place yourself behind them, spells cast by the Hollow Priests will hit them instead of you. This is a good opportunity to heal or cast your own spells.

Melee

This is a strength in numbers boss battle. The best method here is to just go nuts. Attack as many of them as you can. Their poise is low so any strong weapon can break their poise.

Magic

Flame Swathe or Scraps of Life work wonders, as does any area of effect spell. Be patient and unload your spells after the sorcerers cast their spells. Because all enemies in the room are hollow, Soul Appease also works very well.
Well-placed Dark Fogs or Toxic Mists will poison and eventually kill all the enemies.
Sorcerers can use Soul Greatsword as it will consecutively hit most enemies and do a decent amount of damage.